Escalation Functionality - Mailbox only license

[netwizzerd]

Trying to setup a pretty basic escalation profile that sends an email to five email addresses whenever a voicemail is received. This is on a mailbox-only licensed extension.
Doesn't seem to work at all. None of the 5 recipients receive the email notification.
However, if I go and configure the same escalation profile on an 'Extension & Mailbox' licensed extension, all 5 people receive the email.
Anyone know why a 'Mailbox Only' licensed extension would not behave the escalation profile attached to it? The User Group and all the permissions-related settings are the same as the 'Extension & Mailbox' licensed extension just so I have apples-to-apples.

Any thoughts?

Thanks

[netwizzerd]

Just in case anyone else ever experiences the same problem:
Turns out that mailbox-only extensions have the 'Message Notification' checkbox UNCHECKED by default. The checkbox is in the config settings for the various call handling modes.